Drag-along rights let the majority of shareholders (or a defined supermajority) compel the minority to vote for and participate in an approved sale. Without drag-along, a small holdout could block an acquisition or extract a side payment.
The definition of 'majority' varies, some drags require both common and preferred majorities, some require board approval, some require a minimum acquisition price. Drag-along is standard in modern term sheets and rarely contested.